Inductive linear displacement transducers and signal processors adapted for use with the same are well known in the art. See, for example, U.S. Pat. No. 4,667,158, issued May, 19, 1987 to Robert W. Redlich for "Linear Position Transducer And Signal Processor", which discloses an inductive linear displacement transducer and an associated signal processor adapted to measure the straight line motion of an object.
In general, the transducer disclosed in U.S. Pat. No. 4,667,158 comprises a hollow tube-like support bobbin formed out of an electrically insulating material (e.g. plastic); a helical coil wound around the outer surface of the bobbin, the helical coil being formed out of a good electrical conductor (e.g. copper); a cylindrical core axially movable within the bobbin, the cylindrical core being formed out of a good electrical conductor having low magnetic permeability (e.g. copper or aluminum); an attachment rod connecting the core with the object whose motion is to be measured; a shield tube surrounding the remainder of the transducer and confining magnetic flux generated by current flowing through the coil to the interior of the transducer and shielding the coil from stray fields, the shield tube being formed out of a material having both high electrical conductivity and high magnetic permeability (e.g. soft iron or low carbon steel); and a tubular layer disposed between the coil and the shield tube and having the effect of reducing the reluctance of this space to a low value, the tubular layer being formed out of a material having high magnetic permeability and low electrical conductivity (e.g. a ferrite powder distributed within a suitable hardened bonding agent).
Still other details relating to the construction of the inductive linear displacement transducer disclosed in U.S. Pat. No. 4,667,158 are provided in that patent, the contents of which are incorporated herein by reference.
When the attachment rod of the transducer is connected to an object whose displacement is to be measured, and the coil is suitably energized, changes in the position of the object will result in changes in the position of the core relative to the coil; this movement of the core relative to the coil will, in turn, cause changes in the inductance of the coil by means of skin effect, which changes in inductance can be measured to provide an indication of the linear displacement of the object.
U.S. Pat. No. 4,667,158 also discloses signal processor means for measuring such changes in inductance so as to indicate the linear motion of the object. Such signal processor means make use of a bridge circuit adapted to provide an output voltage which is proportional to the distance moved by the core (and hence proportional to the distance moved by the object connected to the core). More specifically, a source of AC voltage in the range of 50-200 kHz excites the bridge circuit, one branch of which is the transducer coil. Half-wave rectifiers are used to convert the AC output signal from the bridge circuit to a DC voltage which is proportional to, or otherwise varies as a function of, the distance moved by the core.
Still other details relating to the construction of the signal processor means disclosed in U.S. Pat. No. 4,667,158 are provided in that patent, the contents of which have been incorporated herein by reference.
Unfortunately, it has been found that the linear displacement transducer and signal processor means disclosed in U.S. Pat. No. 4,667,158 are affected by temperature, to the extent that changes in the transducer's temperature can cause changes in its output signal which are not related solely to changes in its core position. Stated another way, it has been found that changes in transducer temperature can cause the transducer's output signal to vary, so that it is no longer just proportional to the distance moved by the core.
More specifically, the curve C.sub.1 in FIG. 1 schematically illustrates the output signal of a typical inductive linear displacement transducer of the type described above, when the transducer is at some temperature T.sub.1 and its core is set in its "out" (i.e., extended) position. The curve C.sub.2 in FIG. 1 schematically illustrates the output signal of the same transducer when the transducer is at the same temperature T.sub.1 but its core is set in its "in" (i.e., retracted) position. As can be seen by comparing the curves C.sub.1 and C.sub.2 in FIG. 1, a change in the transducer's core position while holding its temperature constant has the effect of changing the amplitude component of the signal coming from the transducer, but it has essentially no effect on the frequency or the DC offset component of that signal. As a result, so long as the temperature of the transducer remains stable, the AC output signal coming from the transducer can be read using half-wave rectifiers so as to provide a corresponding DC voltage which is representative of the transducer's core position at any point along the limits of its stroke.
As noted above, curve C.sub.1 in FIG. 1 schematically illustrates the output signal of a typical inductive linear displacement transducer when that transducer is at some temperature T.sub.1 and its core is set in its "out" position. The curve C.sub.1 from FIG. 1 has been duplicated in FIG. 2. The curve C.sub.3 in FIG. 2 schematically illustrates the output signal of the same transducer when the transducer is at a different, higher temperature T.sub.2 and its core is set in its "out" position. (It is to be appreciated that the curves C.sub.1 and C.sub.3 have been exaggerated to some extent in FIG. 2 to better illustrate the effect being observed). As can be seen by comparing the curves C.sub.1 and C.sub.3 in FIG. 2, a change in the transducer's temperature while holding its core position constant has the effect of changing the amplitude component of the tranducer's output signal.
(It is also to be appreciated that a change in the transducer's temperature can also cause a change in the DC offset component of the transducer's output signal. However, such a change in the DC offset component does not create a problem with linear displacement transducers and signal processor means of the sort disclosed in U.S. Pat. No. 4,667,158, since any such change in the DC offset can be easily removed from the transducer's output signal by filtering and hence does not negatively affect the accuracy of the system. In this regard, it is also to be appreciated that any such changes occurring in the DC offset component due to temperature effects on the transducer have been omitted from FIG. 2 for purposes of clarity so as to help emphasize the change taking place in the AC component of the transducer's output signal, which is the effect of interest.)
In view of the fact that (a) variations in core position cause changes in the amplitude of the AC component of the transducer's output signal, and (b) variations in transducer temperature cause changes in the amplitude of the AC component of the transducer's output signal, it will be seen that signal processor means of the sort adapted to look principally at changes in the amplitude component of the transducer's output signal to identify changes in the transducer's core position will be inaccurate when changes occur in the temperature of the transducer. Stated another way, inasmuch as both variations in core position and variations in transducer temperature can cause changes in the amplitude of the transducer's output signal, signal processor means adapted to look generally at changes in the amplitude component of the transducer's output signal will have no way of determining whether a detected change in amplitude is occurring due to a change in core position or a change in transducer temperature. Hence, use of an inductive linear displacement transducer with such signal processor means in a temperature-varying environment may render the system inaccurate as a reliable measure of changes in core position.
In some circumstances the aforementioned temperature effects on the inductive linear displacement transducer and its associated signal processor means may be relatively insignificant and may be safely ignored. This will be better appreciated when it is recalled that the curves C.sub.1 and C.sub.3 in FIG. 2 are exaggerated relative to one another for better illustration of the effects being discussed; in fact, in a typical inductive linear displacement transducer application, the amplitude component of the transducer's output signal varies only about 0.5-1.0% over a range of approximately 100.degree. F. However, for some applications these temperature effects can be significant and can render the system sufficiently inaccurate as to be unreliable for its intended purpose.
In view of the foregoing, efforts have been made to compensate for such temperature effects on the inductive linear displacement transducer and its signal processor means.
In one arrangement, the known temperature coefficient of resistance ("TC") of the transducer's coil is used to detect and compensate for changes in temperature. More particularly, the change in the DC offset of the transducer's output signal is correlated to changes in the temperature of the transducer; this information is then used to correct the transducer's output signal for changes in temperature. Unfortunately, however, this approach tends to be relatively crude, for several reasons. For one thing, the DC temperature sensitivity of the coil tends to be fairly low. More specifically, the linear displacement transducer typically operates at about 25 mA and has a DC resistance of about 2 ohms, so that the transducer's winding has a DC voltage drop of about 50 mV. At a temperature sensitivity of approximately +3900 ppm/.degree.C., this translates into a transducer temperature coefficient ("TC") of only about +195 uV/.degree.C. at the transducer's operating levels. In addition, since this technique requires that the DC resistance of the transducer's connecting cable be read along with (i.e., in additive relation to) the DC resistance of the transducer winding, and since the DC resistance of the transducer's connecting cable is fairly close to the DC resistance of the transducer winding (typically about 2 ohms for the transducer's winding and about 1 ohm for the normal 10 foot connecting cable), it will be seen that changes in the DC offset may not necessarily reflect just the temperature changes taking place at the transducer; they may also reflect temperature changes taking place in the connecting cable. This can be a serious consideration, especially when one takes into account the fact that the transducer's connecting cable is typically about 10 feet long, and hence is quite likely to be at a different temperature (or temperatures) than the transducer's coil. On account of the foregoing, it will be seen that attempts to use the DC temperature sensitivity of the transducer's coil to correct for changes in transducer temperature tend to yield relatively poor results.
In another arrangement, the known temperature coefficient ("TC") of a conventional silicon diode is used to detect and compensate for changes in transducer temperature. More particularly, the transducer's coil is connected between a source of AC voltage and a common line, and a conventional silicon diode is connected between the common line and a third terminal, and a temperature-compensating signal is then read from the third terminal. Since a conventional silicon diode typically has a temperature coefficient ("TC") of about -2.07 mV/.degree.C. at the transducer's operating levels, the silicon diode is approximately 10 times more sensitive than the transducer's winding in detecting changes in temperature. Unfortunately, however, this technique requires the provision of a third wire, and in many circumstances there is a need to hold the number of wires down to two.